1. Field of the Invention
The present invention relates to the inclusion of field shields in Schottky barrier devices, and more particularly, to the inclusion of one or more field shield diffusions at the metal-semiconductor interface to reduce the surface electric field along the interface, thereby decreasing the reverse bias leakage current of Schottky barrier devices.
2. Description of the Prior Art
Schottky barrier (metal-semiconductor) devices, in particular, diodes, are often used in circuits because they have a low forward voltage drop and a very fast reverse recovery time. These properties make Schottky diodes very useful in applications such as high-speed switching power supply rectifiers. However, compared with conventional p-n junction diodes, Schottky barrier diodes exhibit poor reverse bias characteristics manifested in an increased leakage, particularly at voltages approaching breakdown voltage.
In the past, the reverse characteristics of Schottky barrier diodes were improved by increasing the breakdown voltage of the device, utilizing p-type guard rings diffused into the n-type semiconductor material, as disclosed in U.S. Pat. No. 3,541,403 issued to M. P. Lepselter et al on Nov. 17, 1970. As disclosed, the guard ring is located in the substrate under the insulator-metal interface and functions to reduce the edge breakdown effects existing at the intersection of this interface and the semiconductor surface. The same guard ring structure is discussed in an article entitled "Silicon Schottky Barrier Diode with Near-Ideal I-V Characteristics" by M. P. Lepselter et al appearing in Bell System Technical Journal, Vol. 47, No. 2, pp. 195-208.
An improved method for forming guard rings in Schottky barrier diodes is disclosed in U.S. Pat. No. 4,119,446 issued to S. T. Mastroianni on Oct. 10, 1978. Here, the metal-semiconductor structure is formed first and the metal is then used in conjunction with another mask to form a guard ring self-aligned with the periphery of the metal.
Although the use of guard rings will improve the reverse characteristics of Schottky barrier diodes by reducing the edge breakdown effects, relatively large leakage current in the reverse blocking mode will still exist, due to the presence of a high surface electric field along the planar metal-semiconductor interface away from the edge of the interface. This leakage current generally increases very rapidly as the reverse potential is increased and may be several orders of magnitude larger than the leakage current of a diffused junction diode when the electric field approaches the silicon avalanche limit.
In order to reduce the Schottky barrier diode reverse leakage current, a Schottky metal (or metal silicide) which has a high barrier potential can be utilized. Although this will improve the reverse characteristics, the high barrier potential results in a higher forward voltage drop and, therefore, greater power dissipation than desired. In an alternative method, the electric field is reduced at the Schottky barrier when the device is under reverse bias, which results in reducing the leakage current. In particular, the electric field is reduced by increasing the resistivity and depth of the N-type silicon cathode (for the case of a metal-N silicon diode). However, this method of decreasing the leakage current will result in an increased series resistance between the anode and the cathode and thus will again result in an increased forward voltage drop. Further, this method is not very desirable in high-voltage integrated circuit technology since the N-type cathode material may also be used as collectors or drains of bipolar or MOS transistors, respectively, and the increased resistivity will adversely affect the characteristics of these devices.
There remains to be solved the problem of eliminating the leakage current present in Schottky barrier devices related to the presence of a surface electric field without unnecessarily increasing the forward voltage drop of the device.